compact: Clarify mutability restriction
compact: Clarify mutability restriction

Fixes #13508.

[skip ci]

Test Plan: Read it

Reviewers: austin

Subscribers: rwbarton, thomie

Differential Revision: https://phabricator.haskell.org/D3407

ezyang/ghc,ezyang/ghc,sdiehl/ghc,ezyang/ghc,sdiehl/ghc,sdiehl/ghc,shlevy/ghc,shlevy/ghc,ezyang/ghc,sdiehl/ghc,shlevy/ghc,shlevy/ghc,ezyang/ghc,shlevy/ghc,shlevy/ghc,sdiehl/ghc,ezyang/ghc,sdiehl/ghc,ezyang/ghc,sdiehl/ghc,shlevy/ghc